6. Recommendations

Observe that the goal of hierarchical routing in the Internet is not to reduce the total amount of routing information in the Internet to the theoretically possible minimum, but just to contain the volume of routing information within the limits of technology, price/performance, and human factors. Therefore, organizations that could provide reachability to a sufficiently large fraction of the total destinations in the Internet and could express such reachability through a single IP address prefix could expect that a route with this prefix will be maintained throughout the default-free part of the Internet routing system, regardless of where they connect to the Internet. Therefore, using the "address ownership" policy when allocating addresses to such organizations is a reasonable choice. Within such organizations this document suggests the use of the "address lending" policy.

For all other organizations that expect Internet-wide IP connectivity, the reachability information they inject into the Internet routing system should be subject to hierarchical aggregation. For such organizations, allocating addresses based on the "address ownership" policy makes hierarchical aggregation difficult, if not impossible. This, in turn, has a very detrimental effect on the Internet routing system. To prevent the collapse of the Internet routing system, for such organizations, this document recommends using the "address lending" policy. Consequently, when such an organization first connects to the Public Internet or changes its topological attachment to the Public Internet, the organization eventually needs to renumber. Renumbering allows the organization to withdraw any exceptional prefixes that the organization would otherwise inject into the Internet routing system. This applies to the case where the organization takes its addresses out of its direct provider's block and the organization changes its direct provider. This may also apply to the case where the organization takes its addresses out of its indirect provider's block, and the organization changes its indirect provider, or the organization's direct provider changes its provider.

Carrying routing information has a cost associated with it. This cost, at some point, may be passed back in full to the organizations that inject the routing information. Aggregation of addressing information (via CIDR) could reduce the cost, as it allows an increase in the number of destinations covered by a single route. Organizations whose addresses are allocated based on the "address ownership" policy (and thus may not be suitable for aggregation) should be prepared to absorb the cost completely on their own.

Observe that neither the "address ownership," nor the "address lending" policy, by itself, is sufficient to guarantee Internet-wide IP connectivity. Therefore, we recommend that sites with addresses allocated based on either policy should consult their providers about the reachability scope that could be achieved with these addresses, and associated costs that result from using these addresses.

If an organization doesn't require Internet-wide IP connectivity, then address allocation for the organization could be done based on the "address ownership" policy. Here, the organization may still maintain limited IP connectivity (e.g., with all the subscribers of its direct provider) by limiting the distribution scope of its routing information to its direct provider. Connectivity to the rest of the Internet can be handled by mediating gateways (e.g., application layer gateways, Network Address Translators (NATs)). Note that use of mediating gateways eliminates the need for renumbering, and avoids burdening the Internet routing system with non- aggregatable addressing information; however they have other drawbacks which may prove awkward in certain situations.

Both renumbering (due to the "address lending" policy), and non- aggregated routing information (due to the "address ownership" policy), and the use of mediating gateways result in some costs. Therefore, an organization needs to analyze its own connectivity requirements carefully and compare the tradeoffs associated with addresses acquired via either policy vs. having connectivity via mediating gateways (possibly augmented by limited IP connectivity) using addresses acquired via "address ownership." To reduce the cost of renumbering, organizations should be strongly encouraged to deploy tools that simplify renumbering (e.g., Dynamic Host Configuration Protocol [RFC 1541]). Use of the DNS should be strongly encouraged.